Information recording medium, information recording method and information recording apparatus

ABSTRACT

A recording medium having a dummy signal area on at least one side of a recording area is used and information is recorded with a dummy signal added to at least one of the leading and trailing sides in the recorded information signal. The user data area remains unaffected by repeated overwriting so that recorded user data is reproduced correctly. In particular, the wave form distortion caused by fluidization of a recording film remains at the dummy signal area and does not affect the user data area, so that the reproduction of the target information (user data) is assured.

This is a continuation application of U.S. Ser. No. 09/963,366, filedSep. 27, 2001, now U.S. Pat. No. 6,385,153; which is a continuation ofU.S. Ser. No. 09/629,867, filed Aug. 1, 2000, now U.S. Pat. No.6,317,408; which is a continuation application of U.S. Ser. No.09/412,787, filed Oct. 5, 1999, now U.S. Pat. No. 6,111,850; which is acontinuation application of U.S. Ser. No. 09/188,370, filed Nov. 10,1998, now U.S. Pat. No. 5,974,023; which is a continuation applicationof U.S. Ser. No. 08/892,657, filed Jul. 15, 1997, now U.S. Pat. No.5,878,021; which is a continuation application of U.S. Ser. No.08/678,070, filed Jul. 10, 1996, now U.S. Pat. No. 5,703,867.

FIELD OF THE INVENTION

The present invention relates to an information recording medium, aninformation recording method and an information recording apparatuswhich are capable of recording such information in real time by means ofrecording beams, for example, laser beams as digital informationincluding pulse-frequency modulated analog signals of projected imagesand voice, data in electronic computers, facsimile signals and digitalaudio signals. More particularly, the present invention relates to adisk shaped recording medium using an erasable, phase-change typeoptical recording film.

BACKGROUND OF THE INVENTION

Conventional phase-change type optical disks have a recording filmcapable of fast erasure by crystallization that is materialized within atime substantially equal to the time required for a recording laser beamto pass on a point on a disk, for example. The conventional method ofrecording and erasing information to and from such an erasable recordingfilm is to change the power of the laser beam between two levels higherthan the reading power levels, these levels are a high power level andan intermediate power level. The advantage of this method is that theso-called overwriting (replacing old information with new without priorerasing) is possible. When such overwriting is repeated a plurality oftimes, however, the laser beam irradiates the recording film repeatedlyin accordance with the recording information signal. This results in ahigh probability of allowing VFO recording marks to be repeatedlywritten to the same place in an area close to a preformat section inparticular, and therefore variations in film thickness tend to occurlocally because of recording film fluidization.

According to Japanese Unexamined Patent Publication No. 150725/1991, amethod that avoids the recording film from being repeatedly written atthe same place involves shifting the data write start position each timedata is written or at appropriate times when old data is overwritten,whereby mark-forming positions are properly distributed. Thus theoverwriting cyclability of the optical disk are improved.

SUMMARY OF THE INVENTION

Even when information is recorded on the phase-change type optical diskas in the prior art, the fluidization of the overall recording filminevitably occurs as overwriting is repeated a plurality of timesbecause the temperature distribution in the laser beam is asymmetrical.Consequently, a change in the thickness of the recording film is broughtabout in that the film becomes thick on one side and thin on the otherside (the initial or terminal end portion of a recording area taken fromthe perspective of the direction of recording) or the film becomes thickat the center portion of the recording area and thin at the starting andending areas, so that the distortion of the reproducing signal waveformalso occurs. The latter type of thickness change is more difficult tostop. Therefore, there arises a problem of making the area having thewaveform distortion expand as the number of overwriting times increases.In the case of an erasable digital video disk, moreover, mark edgerecording may be used with both edges of a recording mark provided withinformation for the purpose of increasing density. When such a mark edgerecording system is employed, deterioration due to fluidization isfurther evident because a long mark is repeatedly recorded.

An object of the present invention intended to solve the foregoingproblems in the prior art is to provide an information recording medium,an information recording method and an information recording apparatusthat ensure reproduced signals properly corresponding to recordedinformation signals.

According to the present invention, a recording medium capable ofrecording or overwriting information with a laser beam is provided. Therecording medium has a preformatted area in which information such as atrack address has been formed with pits or with recording marksresulting from a phase change. In addition to the preformatted area, agap area, a recordable area such as a VFO/SYNC area and a user data areaare reserved. Moreover, a dummy area is provided on at least one of thesides before and after the recording area. A part of the VFO area can beused instead of the front dummy area, by making the VFO area long.Further, a buffer area to allow for a margin for the rotational jitterof a motor is located just after the dummy area. In this case, one ofthe objectives for the provision of the dummy area is to preventinformation actually read from the user data area from being damagedeven though repeated overwriting causes the fluidization of therecording film. Such damage is prevented because the distortion of areproduced signal waveform is confined to the VFO/SYNC area and/or thedummy area. Further, the dummy area is preferably set to be longer thanthe maximum shift width of a position where the writing of a recordedinformation signal is started. Further, it is preferable to add a pitarray (data area postamble) depending in accordance with applicablestandards before this dummy area to avoid the mark and the space frombeing inverted.

According to the invention, recording is made by adding a dummy signalto at least the front or rear side of a target (recorded) signal such asVFO, SYNC and user data. It is preferable to add the dummy signal to therear side (the trailing side with respect to the direction of recording)when a dummy signal is added to only one side of the target signal. Inthis case, the dummy signal is chiefly recorded in the dummy area.

Further, according to the invention, the fluidization of the recordingfilm resulting from repeated overwriting can be moderated by setting theaverage energy of the laser beam during dummy signal recording to belower than that during the target information signal recording.Consequently, the distortion of the reproduced signal waveform due tothe fluidization slightly occurs in the dummy area where the dummysignal is recorded and in part of the buffer area. However, therecording area where the target signal is recorded remains unaffected bythe fluidization to ensure that a reproduced signal correctlycorresponding to the target signal is obtained. Collectively, the sum ofthe dummy area and the buffer area can be called a buffer area.

For the recording of the dummy signal, the average energy thereof may belowered, for example, by adjusting the duty ratio of a single-frequencysignal. In a mark edge recording system, the duty ratio of the dummysignal may be set to not greater than 50% because the average duty ratiois about 50%. The duty ratio of at least a part of the dummy signal ispreferably in a range of at least 10% to about 40%, and more preferablyin a range of at least 20% to about 30%. Moreover, the average energy ofthe laser beam during the recording of the dummy signal may be decreasedstepwise or linearly by adjusting the duty ratio of the dummy signalstepwise or linearly; or otherwise the recording power may be varied. Itis more preferred to vary the duty ratio of the dummy signal than tovary the recording power in order to simplify the apparatus. The dummysignal may be added to either the front or rear side of the recordinginformation signal, that is mainly VFO, SYNC, and user data, dependingon the recording media characteristics. In the preferred case of addingthe dummy signal to only one side, the dummy signal is added to only therear side of the recorded information signal rather than the front sidewhere the VFO has a function of the dummy signal.

The distortion level of the reproduced signal waveform can also belowered by shifting the start position of writing the recordedinformation signal to effect overwriting. In this case, the maximumshift width is preferably set to be shorter than the dummy signal thatis added; particularly, it is preferably set to be approximately{fraction (1/20)} to ½ of the length of the recorded dummy signal.

Also according to the present invention the recording medium storesthereon information concerning the pattern of the dummy signal that isused, for example the duty cycle of the dummy signal. For example, it ispreferred that such information be recorded in the control data area ofthe recording medium.

Recording films utilizing a phase change such as recording films of aGe—Sb—Te system and a Ag—In—Sb—Te system may be used according to thepresent invention. The use of a recording film containing a high-meltingpoint material such as Cr—Te and Ag—Te whose melting point is higherthan that of the main component material and a recording medium with anSi/metal double reflective layer is preferred because the recording filmthickness is further restrained from varying because of fluidization.

Still further, according to the present invention, information isrecorded by an information recording apparatus including: a laser beamsource, an optical system, automatic focusing (AF) means, trackingmeans, reproduced signal detection means, means to read information in acontrol data area, a recording medium on which information on a patternof a dummy signal to be added to at least one of the leading or trailingsides of a recorded information signal is recorded beforehand, means forrotating or moving the recording medium, means for condensing laserbeams from the laser beam source on the recording medium, signalmodulating means for converting a signal to be recorded into amodulation code, means for adding the dummy signal as set forth herein,recording waveform generation means for generating a recording waveformcorresponding to the recording code with the dummy signal added,recording start position control means for shifting a recording startposition at random (preferably wherein the maximum shift width of therecording start position, for example, expressed in a unit of Bytes isshorter than the length of the dummy signal expressed in a unit ofBytes), laser driving means for driving the laser beam source inaccordance with the recording waveform, means for converting theintensity change of the laser beam reflected from the recording mediuminto an electrical signal, means for amplifying the reproducedelectrical signal, binary conversion means for converting the electricalsignal into a binary waveform, means for deleting the dummy signal, andmeans for decoding the binary signal to make the decoded signal aninformation signal (user data).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial sectional view of a disk embodying the presentinvention.

FIG. 2 is a diagram of an exemplary sector format according to anembodiment of the invention.

FIG. 3 is a block diagram of an information recording apparatusaccording to an embodiment of the invention.

FIG. 4 is a flow chart showing the steps followed in generating thedummy data by reading information on the dummy data pattern from a diskafter the disk is inserted in the apparatus of the invention;

FIG. 5 is a diagram of an example of the data area postamble and thedummy data according to the invention; and

FIGS. 6(a) and 6(b) are diagrams of examples of the dummy data patternaccording to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A detailed description will subsequently be given of the preferredembodiments of the present invention.

The recording film for use in the present invention is preferably acrystal-amorphous phase-change optical recording film capable of fastcrystallization, a recording film utilizing an amorphous-amorphouschange, a crystal-crystal phase-change (e.g. a change in the crystalsystem and particle size) recording film or a magneto-optic recordingfilm. However, any other recording film which has a tendency to havewaveform distortion at the beginning and/or the end of a recording areamay also be used.

In addition to the preformatted area and a gap area, a recording areasuch as the VFO/SYNC area and the user data area capable of recordinginformation are provided. Moreover, the dummy area is provided on atleast one of the leading or trailing sides located respectively beforeand after the recording area, and this dummy area serves to preventinformation actually read from the data area from being damaged eventhough repeated overwriting causes the fluidization of the recordingfilm. The damage is prevented because the distortion of the regenerativesignal waveform resulting from the fluidization thereof is confined tothe VFO area, the dummy signal and/or the buffer area.

A description will subsequently be given of the functions in a casewhere a recording medium is used in which forward fluidization (therecording film is fluidized in the same direction as the direction inwhich the laser beam proceeds along the surface of the disk) is causedtherein by a plurality of overwritings. The sector format of therecording medium used herein includes the preformatted area, the gaparea, the front dummy area, the recording data area, the rear dummy areaand the buffer area arranged in this order. The recording data area isformed with the VFO/SYNC area and the user data area. At this time, theshift width at the recording start position is made shorter than thelength of the dummy area to ensure that the recorded information signalis correctly reproduced. When overwriting is repeated a plurality oftimes on the recording medium, fluidization occurs and the fluidizedportion of the recording film moves from the beginning or leading sideto the trailing or ending side of the recording area. Therefore, thereproduced signal level varies.

In this recording medium, a reflectance first decreases, then increasesas the recording film becomes thinner and increases as it becomesthicker. In other words, the reflectance first decreases then increaseson the leading side where the recording film becomes thin, whereas itincreases on the trailing side where the recording film becomes thick.Therefore, the distortion of the reproduced signal waveform occurs onthe leading and trailing sides of the recording area, thus causingerrors. However, the error generation resulting from repeatedoverwriting can be eased by adding the dummy signal to at least one ofthe leading or trailing sides respectively before or after a targetinformation signal such as the VFO/SYNC and data signal recording areaand by making the average energy of the dummy signal lower than that ofthe recorded user data signal. At the time of the reproduction then,only the reproduced signal waveform corresponding to the user datasignal is reproduced, whereby the user data recording area remainsunaffected by the fluidization even though the reproduced waveformdistortion in the dummy signal area and/or that of the buffer area occurto ensure that the user data signal is reproduced correctly. Further, nogreat distortion is caused in the reproduced signal waveform of therecorded user data signal even if the number of overwritings increases.

In this case, the average energy of the laser beam during the dummysignal recording may be lowered by, for example, recording the dummysignal with a single-frequency signal and decreasing the duty ratio ofthe signal. In the mark edge recording system, the duty ratio of thedummy signal may be set to not greater than 50% because the average dutyratio of the user data is about 50%. In the case of a disk causing theforward fluidization, the duty ratio of the dummy signal shouldpreferably be decreased further to as low as 25%. Moreover, the averageenergy of the laser beam during the dummy signal recording may bedecreased stepwise or consecutively by adjusting the duty ratio of thedummy signal stepwise or linearly; or otherwise, the laser power levelmay be varied instead of the duty ratio.

Although the average laser beam energy can be decreased by varying thepulse width of the recording waveform and/or the power that is used, itis preferred to vary the duty ratio of the dummy signal because theapparatus is simplified. The dummy signal may be added to either thefront (leading) or rear (trailing) side or both of the recordedinformation signal as occasion demands. In this case, the dummy signalmay be added to only the rear (trailing) side of the recordedinformation signal rather than the front side thereof where the VFOsignal has a similar function as that of the dummy data signal.

Although the invention is disclosed with respect to a disk mediumembodiment any other recording medium in the form of not only a disk butalso a card and the like is applicable to the present invention.

FIG. 1 is a partial sectional view of a disk used according to apreferred embodiment of the invention. First, a ZnS—SiO₂ dielectriclayer 2 about 125 nm thick is formed by magnetron sputtering on apolycarbonate substrate 1 for a continuous tracking servo, the substrate1 having a diameter of 5 inches and a thickness of 0.6 mm. Subsequently,a recording film 3 of Cr₅Ge₂₀Sb₂₀Te₅₅ is formed to about 30 nm thicknesson the ZnS—SiO₂ dielectric layer 2 and additionally a ZnS—SiO₂dielectric layer 4 about 20 nm thick is formed on the recording film 3.Further, an Si layer 5 is formed to about 100 nm thickness on thedielectric layer 4 and then an Al—Ti-alloy reflective layer 6 about 100nm thick is formed on the Si layer 5.

The formation of these film layers is carried out successively by oneand the same sputtering apparatus. Further, anultraviolet-light-hardening resin layer 7 is applied onto the alloyreflective layer 6 and then a hot-melt adhesive layer 8 is used toadhere a protective layer 9 securely onto theultraviolet-light-hardening resin layer 7. In this case, it is possibleto obtain a bonded disk having a recording capacity twice as large asthat of the above bonded laminate by bonding, in place of the protectiveplate 9, another disk as a combination ranging from the polycarbonatesubstrate 1 up to the ultraviolet-hardening resin protective layer 7.

FIG. 2 shows an example of a sector format of the substrate usedaccording to the preferred embodiment of the invention. The sectorformat includes a preformatted area 10 where information such as sectorand track addresses are formed as uneven pits. In addition to thepreformatted area 10 which is a read-only area, a gap area 11 and arecording area 14 (group section) such as a VFO/SYNC area 12 and a userdata area 13 capable of being recorded (including overwriting) withinformation are provided.

According to this embodiment of the invention, moreover, dummy areas 15,16 are provided at the beginning and the end in the recording area,respectively. Further, a buffer area 17 providing a margin for arotational jitter is also provided behind the dummy area 16 whichfollows the recording area. The dummy areas 15, 16 serve to preventinformation that is read from the user data area 13 from being damagedeven though repeated overwriting causes the fluidization of therecording film. This damage is prevented because the distortion of areproduced signal waveform resulting from the fluidization thereof isconfined to only the front-side dummy area 15 before the VFO/SYNC areaand further because the distortion of the reproduced signal waveformresulting therefrom is confine to the rear-side dummy area 16 behind theuser data area. In this case, the dummy areas 15, 16 are set to belonger than the maximum shift width at a position where the writing of arecording signal is started, and the rear-side dummy area 16 whoserecording film thickness varies greatly due to the fluidization is setto be longer than the front-side dummy area 15. According to recordingmedia characteristics, such a dummy area for restraining the effect offluidization may be provided on either front or rear side, andpreferably on the rear side if it is to be provided on only the oneside.

In this embodiment, a dummy signal is added, as an example, to bothsides of a recorded information signal by a time length of 500 Tw (1 Tw:90 ns, wherein Tw represents a reproduced signal detection windowwidth). Further, in this embodiment, a recording start position isshifted at random within a range of 360 Tw. In this case, the dummysignal is mainly recorded in the dummy area. In order to moderate thefluidization of the recording film resulting from repeated overwriting,moreover, the average energy of the dummy signal is set to be lower thanthat of the user data signal; in other words, affected area by thefluidization is made to increase gradually.

In practice, a single-frequency (recording pulse period: 10 Tw) signalis used for the dummy signal and the duty ratio of the signal is setlow. In the case of a mark edge recording method, the duty ratio becomessubstantially equal to 50% in terms of the average energy when therecording is made at the single frequency. Consequently, the duty ratioof the dummy signal is preferably not greater than 50%.

Since the recording film is fluidized in the same direction as thedirection of the laser beam (forward fluidization) due to repeatedoverwriting on the disk of this embodiment according to the invention,the duty ratio is set as low as 30%. The effect of adding the dummysignal was noticeable when the duty ratio was at least 10% and up to andincluding 40%. The effect was especially pronounced when the duty ratiowas in the range of at least 20% up to and including 30%, whereby theeffect of fluidization of the recording film was significantly easedwith the effect of enlarging the recording power margin. Moreover, theaverage energy of the laser beam during dummy signal recording, may,according to various conditions, be decreased stepwise or linearly. Inthis embodiment, the recording laser pulse corresponding to a recordingmark was divided into a plurality of pulses by a recording waveformgenerator. Thus the laser pulse duty was less than 50% even for therecording of the user data.

The reproduced signal waveform distortion due to fluidization has beenreduced by randomly shifting the write start position of the recordedinformation signal by an amount approximately equal to {fraction (1/20)}to ½ of the dummy signal area.

Information on the pattern of the dummy signal (e.g., information on asingle-frequency pattern in which the average recording mark lengthrecorded by the dummy signal becomes shorter than the average spacelength between the recording marks, etc.) is recorded with pitsbeforehand in the control data area of the disk in this preferredembodiment of the invention; thus recording is made according to thisinformation.

Compared to a Ge—Sb—Te or Ag—In—Sb—Te recording film to which nohigh-melting point material such as Cr—Te has been added, a recordingfilm containing a high-melting point material such as Cr—Te and Ag—Te,for example, Ag₂Te is preferred because the recording power margin formoderating the fluidization is attainable to a great extent, though theuse of the recording waveform with the dummy signal added thereto hasthe effect of suppressing the effect of fluidization.

FIG. 3 is an exemplary block diagram of a record playback system in arecord playback apparatus embodying the present invention. Before therecording, dummy data is generated according to information stored inthe control data concerning the dummy data pattern, then the dummy datais stored in a memory 41. At the time of recording, the primary signalto be recorded (user data) is fed into a modulator 18 in which it isconverted into a modulation code. The dummy data generated and stored inthe dummy data memory 41 is read out through the dummy data patternreading circuit 42 and added to the user data with an adder 43. Further,VFO and SYNC signals are added with an adder 44. Also, the pulse signalfrom the adder is passed through a recording start position controller20 for shifting a recording start position of the recording signal atrandom. Then a recording waveform corresponding to a target recordingcode is formed by a recording waveform generator 19 before being outputas recording pulse signals. A laser driver 21 modulates the drivingcurrent of a semiconductor laser 22. Further, a disk 24 is irradiatedwith a condensed laser beam via an optical system in an optical head 23,so that a recording mark is formed. It is understood that although theapparatus is disclosed schematically, it includes elements such as anautomatic focusing (AF) means and tracking means as part of thetransducing system generally referred to as the optical head 23.

At the time of playback on the other hand, the intensity change of thelaser beam reflected from a target address on the disk 24 is received bya light detector 25 in which the laser beam is converted into anelectrical signal. Then the electrical signal is input via a reproducedsignal amplifier 26 to a waveform equalizer 27. Then, the binary signalis formed by a discriminator 28 and decoded by a decoder 29 into a databit string (information). Next, a preformatted area (header) of a targetaddress is detected and analyzed by header reader 33. Then, the dummysignal and header are deleted by dummy signal and header deleter 34, anda reproduced user data signal 35 is obtained.

Next, it is determined if the reproduced signal represents control databy a control-data detector 36. If the reproduced signal contains controldata including information on the dummy data pattern, the dummy datainformation is picked up from the control data by dummy data informationseparator 39. Then, the dummy signal is generated according to theinformation (dummy data) on the dummy data pattern by dummy data patterngenerator 40. If the reproduced signal is not control data, the data isstored in memory 38 through reproduced user data controller 37.

As a modification of the embodiment disclosed with respect to FIG. 3, abit array (data area postamble) can be added by postamble adding circuit43 before the dummy data recorded in the dummy area 16, and theinversion of the ratio of the recording mark and the space for the dummysignal can be avoided. The postamble pattern is generated in accordancewith the user data to be recorded and is added to the user data by adder43 at the position just after the user data in the direction ofrecording.

FIG. 4 shows a flow chart of the steps followed when a disk is insertedin the record playback apparatus of the present invention. First, instep 45, it is detected that the disk is set in the drive by an opticalor mechanical sensor. Then, the optical head is moved to, for example,the innermost area of the disk in step 46 and reading information startsin step 47. In step 48, the reproduced signal is read out and amplifiedfrom the rotating disk, and then the wave form generated thereby isequalized, digitized and decoded in steps 49-51, respectively. Thecontrol data is extracted by deleting the synchronizing signal, etc. instep 52 and in step 53 the information of the dummy data pattern istaken from the control data. Such information (dummy data) includes, forexample, the specification of the duty ratio of the dummy signal to berecorded being added to the user data. In step 54, the dummy datapattern is generated and stored in step 55 in a memory so that the dummysignal can be added when user data is to be recorded.

An example of the data area postamble is shown in FIG. 5 as 31 and thedummy data as 32. Patterns (a) and (b) show the case where the end ofthe user data is a binary “0”, and patterns (c) and (d) show the casewhere the end of the user data is a binary “1”. Accordingly, patterns(a) and (c) show the case where the end of the user data is a mark.Patterns (b) and (d) show the case where the end of the user data is aspace. It is preferable if the postamble is provided according to theknown signal modulation standard.

An example of recording the information on the dummy data pattern in thecontrol data area is shown in FIG. 6(a). The dummy signal area isdivided into two sub areas, and the dummy signal pattern informationarea in the control data area is also divided into two sub areas; eachsub area being divided into three sections. A mark length, expressed inunits of Tw, is recorded using 4 bits in the first section. A spacelength, which is expressed in units of Tw, is recorded using 4 bits inthe second section. The number of repetitions of these marks and spacesis recorded using the following 8 bits (1 byte). Then, a mark length, aspace length, and the number of repetitions in the second sub area isrecorded using 16 bits. Thus, the whole pattern of the dummy dataexpressed by marks and spaces is represented by 4 bytes of control data.

In other examples, a single repetition pattern of a mark and a space canbe written in the dummy data pattern information area, or 3 patterns canbe written dividing the dummy data area into 3 sub areas. When the dummydata area is divided into more than 4 areas, the required space torepresent the patterns in the control data area exceeds 4 bytes.

As another example, the basic mark and space arrangement, which isrepeated in the first sub area of two sub areas in the dummy data area,is recorded in the control data area using 2 bytes in the same manner asin the dummy data area, and the basic mark and space arrangement in thesecond sub area is recorded using the following 2 bytes as shown in FIG.6(b). In this method, 4 bytes are used in the control data area torepresent the pattern in the dummy data when the number of sub areas istwo, and more than 4 bytes are used when the number of sub areas aremore than 3 just like the former example.

Although a description has been given of the continuous servo system byway of example, the same effect is achievable with a sampled servosystem in a like manner as set forth herein. Further, although a laserbeam has been disclosed for recording, it is possible to use an electronbeam or an ion beam instead of the laser beam in accordance with thepresent invention.

We claim:
 1. An information recording apparatus for recordinginformation by energy beam irradiation, comprising: a laser beam sourcefor producing a laser beam; a laser driver to drive the laser beam; arecording waveform generator to generate the recording waveform; meansfor recording a recording signal and a dummy signal added to at least aleading or trailing side of the recording signal, wherein the length ofsaid trailing side dummy signal is longer than that of said leading sidedummy signal, wherein a duty ratio of the dummy signal is not greaterthan 50%, and wherein a recording start position of a combined recordingsignal and dummy signal is shifted at random; and means for adding apostamble pattern after a user data.
 2. An information recordingapparatus according to claim 1, wherein a maximum shift width of therecording start position is shorter than the length of the leading ortrailing side of the recording signal.
 3. An information recordingapparatus according to claim 1, wherein a duty ratio of the dummy signalis not greater than 25%.
 4. An information recording apparatus accordingto claim 1, wherein the maximum shift width of the recording startposition is in range of ½ to ½ of the length of the leading or trailingside of the recording signal.
 5. An information recording apparatus asclaimed in claim 1, wherein said postamble pattern shows whether the endof the user data is a binary “0” or “1”.
 6. An information recordingapparatus as claimed in claim 1, wherein said postamble pattern showswhether the end of the user data is a mark or a space.